This invention relates to improvements in folded optical systems, such as the modular optical systems used in today's intruder detection devices of the optical or passive infrared variety.
Folded modular optical systems are known in the intruder detecting art for focusing infrared (IR) energy, emanating from a would-be intruder in any one of a plurality of different fields of view, onto an IR-sensitive detector. In one such optical system, shown in FIGS. 1 and 2, and presently used in assignee's passive IR device known as the Model DS 964 PIR Detector, a somewhat elongated rectangularly shaped spherical reflector 10, arranged with its optical axis in a vertical plane, cooperates with a unitary, multifaceted optical element 12 to provide a plurality of fields of view V1-V4, one above the other in a substantially vertical plane. As shown, the multifaceted element comprises a plurality of elongated planar reflectors 14, each being arranged at a different tilt angle relative to the horizontal plane. Radiation emanating from the different fields of view strikes the planar reflectors and is redirected thereby toward the spherical element which, in turn, focuses such radiation on the detector D.
In the folded optical system briefly described above, the rectangularly shaped spherical reflector is arranged so that its longer dimension L is parallel to the longer dimension 1 of the planar reflectors. While this arrangement affords a compact detector housing (viewed from front to back), it limits the degree to which radiation can be sharply focused onto the IR detector. Since each planar reflector directs rediation across the entire length of the reflector, the effects of spherical aberration are accentuated. While the effects of such aberrations are not great, they do diminish the sensitivity of the system and should be avoided to the extent economics allow. An obvious solution would be to replace the spherical reflector with one of parabolic shape. This approach, however, adds significant cost to the system and is undesirable if only from that standpoint.
Another disadvantage of the prior art system noted above is that each of the planar reflectors is substantially the same size as the others and, hence, each has the same light-gathering power. This leads to system sensitivity variations as a function of distance over which each field of view is intended to provide coverage. While this uniformity of light-gathering power presents no problem with the near fields of view, where the same target can produce a saturated output regardless of which near field it appears, it does lead to sensitivity fall-off in the far fields.